Announced today just a few hours ago, Niantic Inc. posted on their Pokemon GO Live blog announcing the soon-to-be release of the second Mythical Pokemon in their storied franchise, #251 Celebi! Known from the Anime series as, the "Voice of the Forest," this Mythical follows in a similar design pattern as the first Mythical Pokemon in the series, Mew, as they are both small in stature, but pack a mean punch.
Along with its similar body type, Celebi will also share the same way of unlocking it through "Special Researches," an extensive task list that will eventually yield a way for trainers to catch Celebi. There is no word on what types of tasks are in store this time around, but there were 7 steps, each with 3 tasks (totaling to 21 tasks), to unlock the chance to catch Mew, so expect something similar.
For those who haven't completed their Mew Special Research, fret not, because both can be done concurrently.
Also further down in the post, they mention that Pokemon from the Johto region will spawn more frequently, which will hint that all of Generation 2, and a mixture of some Generation 1 Pokemon, will pop up more freely.
Lastly, the official date was announced for Celebi's Special Research tasks, which will begin on Monday, August 20th probably around 1 PM PDT, as that is usually when they do server side updates.